1. Field of the Invention
The present invention relates to a polarized wave separating structure, a radio wave receiving converter (LNB, Low Noise Blockdown Converter) receiving radio wave from a broadcasting satellite, a communication satellite or the like, and to an antenna apparatus.
2. Description of the Background Art
Microwaves used in satellite broadcasting generally include two components. As a representative, for example, circularly polarized waves include dextrorotatory polarized wave (hereinafter referred to as d-polarized wave) and a levorotatory polarized wave (hereinafter referred to as d-polarized wave).
Therefore, a receiving converter receiving radio waves from satellite broadcasting employs a polarized wave separating structure for separating these two components. Particularly when one of the two components (for example, d-polarized wave only) is to be received, higher degree of separation (degree of cross polarization discrimination) is attained by separating the components by the polarized wave separating structure and absorbing the other component (cross polarization component).
A first prior art example of the polarized wave separating structure of the receiving converter will be described with reference to FIG. 43, which is an exploded perspective view of the main part schematically representing the structure, and FIG. 44, which is a cross sectional view taken along the line XXXXIV—XXXXIV of FIG. 43.
On one side of a substrate 103 having a pair of radio wave receiving probes 104a and 104b, a waveguide 101 is arranged. In waveguide 101, there is formed a stepped waveguide septum 101a parting the inside of waveguide 101 into two. On the other side of substrate 103, a radio wave reflecting portion 102 is arranged. In the radio wave reflecting portion 102, there is formed a radio wave reflecting portion septum 102f parting the inside of radio wave reflecting portion 102 into two. On an end surface of radio wave reflecting portion 102 positioned opposite to substrate 103, a radio wave reflecting surface 102a is formed.
On that side of substrate 103 on which radio wave reflecting portion 102 is positioned, a grounding surface (pattern) 105 is provided, along and in contact with end surfaces of radio wave reflecting portion 102 and radio wave reflecting portion septum 102f. On that surface of substrate 103 on which wave guide 101 is positioned, a grounding surface (pattern, not shown) is formed along and in contact with end surfaces of wave guide 101 and wave guide septum 101a.
Grounding surface 105 in contact with radio wave reflecting portion 102 and the grounding surface in contact with waveguide 101 are electrically connected by means of through holes 106. Thus, waveguide 101 and radio wave reflecting portion 102 are kept, by means of the substrate 103, at the ground potential.
The pair of radio wave receiving probes 104a and 104b are formed on that side of substrate 103 on which radio wave reflecting portion 102 is formed. Wiring portions of radio wave receiving probes 104a and 10b are electrically insulated from grounding surface 105, radio wave reflecting portion 102 and waveguide 101.
By waveguide septum 101a and radio wave reflecting portion septum 102f, the inside of waveguide 101 and the inside of radio wave reflecting portion 102 are parted into two waveguide spaces. The circularly polarized wave entering waveguide 101 is separated by the stepped waveguide septum 101a to linearly polarized wave components, and guided to respective waveguide spaces.
In the first prior art example, in order to prevent leakage of radio wave inside waveguide 101 or radio wave reflecting portion 102 to the outside, or to reduce noise, end surfaces of septums 101a and 102f and of waveguide 101 and radio wave reflecting portion 102 are brought into contact with the grounding surfaces of substrate 103.
Now, waveguide 101 including septum 101a and radio wave reflecting portion 102 including septum 102f are formed by casting technique using, for example, aluminum die cast. Considering dimensional accuracy at the time of actual mass production, it is difficult to bring the end surfaces of septums 101a and 102f and of waveguide 101 and radio wave reflecting portion 102 surely into contact with the grounding surfaces of substrate 103.
More specifically, in the first prior art example, when the end surface of radio wave reflecting portion septum 102a is to be surely brought into contact with grounding surface 105 of substrate 103, it follows that the end surface of waveguide 101 cannot surely be brought into contact with the grounding surface, resulting in a gap at the contact portion. Consequently, it is possible that the radio wave leaks outside, or the noise increases.
In view of the foregoing, a second example has been proposed. The second prior art example will be described with reference to FIG. 45, which is an exploded perspective view of the main portion schematically representing the structure, and FIG. 46, which is a cross sectional view taken along the line XXXXVI—XXXXVI of FIG. 45.
In the second prior art example, an opening 103a is provided in substrate 103, and waveguide septum 101a is extended to pass through the opening 103a of substrate 103. Radio wave reflecting portion septum 102f of the first prior art example is not formed at radio wave reflecting portion 102, and, alternatively, a hole 102i receiving the end surface of the extended waveguide septum 101a is formed.
Further, in the second prior art example, the hole 102i of radio wave reflecting portion 102 is communicated with the outside. Therefore, in order to prevent in-coming/out-going of radio wave from/to the outside, the gap between waveguide septum 101a and hole 102i is sealed by a conductive member 107 formed, for example, of a sheet metal having elasticity.
According to the second prior art example, even when there is a variation in dimensional accuracy at the time of mass production, conductive member 107 deforms, and therefore, it becomes easier to attain sure contact between the entire end surfaces of waveguide 101 and radio wave reflecting portion 102 with the grounding surfaces of substrate 103.
FIG. 47A is a perspective view showing the appearance of conductive member 107 shown in FIG. 46, FIG. 47B is a cross section taken along the line XXXXVIIB—XXXXVIIB of FIG. 47A, and FIG. 47C is a cross section showing a state in which conductive member 107 and septum 101a are attached to hole 102i.
Conductive member 107 will be described with reference to FIGS. 47A, 47B and 47C. Conductive member 107 has an engaging portion 107a that abuts radio wave reflecting surface 102a, and an inward cutout portion 107b having a tip end abutting septum 101a. The width A of FIG. 47A is set to be slightly larger than the width B of hole 102i of radio wave reflecting portion 102 shown in FIG. 47C. Such a structure is to prevent slipping off during assembly, and to attain sure electrical conduction between septum 101a and radio wave reflecting portion 102.
The second prior art example described above, however, has the following problem.
In the second prior art example, a separate conductive member 107 is used. Therefore, cost of the raw material increases, and considering the manufacturing steps, the step of attaching conductive member 107 increases. Thus, the cost is significantly increased.
Further, in the manufacturing step at mass production, it may be possible that attachment of conductive member 107 is unsatisfactory. In such a case, radio wave may be leaked outside through hole 102i or the noise may be increased, and therefore increase of the ratio of defective products and degradation of products are expected. Further, there may be a gap around the cutout portion 107b of conductive member 107, as shown in FIGS. 47A to 47C, and the gap between waveguide septum 101a and hole 102i cannot be sealed by the two side surfaces at which cutout portion 107b is not formed. In other words, it is difficult to actually seal the gap by the structure that employs a separate member to fill the gap between septum 101a and hole 102i, possibly resulting in degradation of product characteristics.